Have you done it yet? If you haven't done it yet, go to this page and follow the
instructions
Download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
change boot order so USB is first, hdd second
boot from installer
follow this guide:
https://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach screen asking for licence,m click "I don't have a key" and win 10 will install and then you will need to go to link above and follow steps to reactivate
If you have already done the upgrade, you will need to go to settings/update & security/activation and there should be contact info on bottom of the page to let you contact MS and explain to them what you did. They might want the email address you registered win 10 with originally or your old licence key... I don't know.